GFS2: Use RCU for glock hash table

This has a number of advantages:

 - Reduces contention on the hash table lock
 - Makes the code smaller and simpler
 - Should speed up glock dumps when under load
 - Removes ref count changing in examine_bucket
 - No longer need hash chain lock in glock_put() in common case

There are some further changes which this enables and which
we may do in the future. One is to look at using SLAB_RCU,
and another is to look at using a per-cpu counter for the
per-sb glock counter, since that is touched twice in the
lifetime of each glock (but only used at umount time).

Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

fs/gfs2/incore.h

@@ -15,6 +15,8 @@
 #include <linux/workqueue.h>
 #include <linux/dlm.h>
 #include <linux/buffer_head.h>
+#include <linux/rcupdate.h>
+#include <linux/rculist_bl.h>
 
 #define DIO_WAIT	0x00000010
 #define DIO_METADATA	0x00000020
@@ -201,7 +203,7 @@ enum {
 };
 
 struct gfs2_glock {
-	struct hlist_node gl_list;
+	struct hlist_bl_node gl_list;
 	unsigned long gl_flags;		/* GLF_... */
 	struct lm_lockname gl_name;
 	atomic_t gl_ref;
@@ -234,6 +236,7 @@ struct gfs2_glock {
 	atomic_t gl_ail_count;
 	struct delayed_work gl_work;
 	struct work_struct gl_delete;
+	struct rcu_head gl_rcu;
 };
 
 #define GFS2_MIN_LVB_SIZE 32	/* Min size of LVB that gfs2 supports */